Communication devices such as terminals are also known as e.g. User Equipments (UE), wireless devices, mobile terminals, wireless terminals and/or mobile stations. Terminals are enabled to communicate wirelessly in a cellular communications network or wireless communication system, sometimes also referred to as a cellular radio system or cellular networks. The communication may be performed e.g. between two terminals, between a terminal and a regular telephone and/or between a terminal and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.
Terminals may further be referred to as mobile telephones, cellular telephones, laptops, or surf plates with wireless capability, just to mention some further examples. The terminals in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another terminal or a server.
The cellular communications network covers a geographical area which is divided into cell areas, wherein each cell area being served by an access node such as a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “eNB”, “eNodeB”, “NodeB”, “B node”, or BTS (Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the terminals within range of the base stations. In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the mobile station. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the mobile station to the base station.
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
3GPP LTE radio access standard has been written in order to support high bitrates and low latency both for uplink and downlink traffic. All data transmission is in LTE controlled by the radio base station.
The development of the 5th Generation (5G) access technology and air interference is still very premature but there have been some early publications on potential technology candidates. A candidate on a 5G air interface is to scale the current LTE, which is limited to 20 Mega Hertz (MHz) bandwidth, N times in bandwidth with 1/N times shorter time duration, here abbreviated as LTE-Nx. A typical value may be N=5 so that the carrier has 100 MHz bandwidth and 0.1 millisecond slot lengths. With this scaled approach, many functions in LTE can be re-used in LTE-Nx, which would simplify standardization effort and allow for a reuse of technology components.
The carrier frequency for an anticipated 5G system could be much higher than current 3G and 4th Generation (4G) systems, values in the range 10-80 Giga Hertz (GHz) have been discussed. At these high frequencies, an array antenna may be used to achieve coverage through beamforming gain, such as that depicted in FIG. 1. FIG. 1 depicts a 5G system example with three Transmission Points (TPs), Transmission Point 1 (TP1), Transmission Point 2 (TP2), Transmission Point 3 (TP3) and a UE. Each TP utilizes beamforming for transmission. Since the wavelength is less than 3 centimeters (cm), an array antenna with a large number of antenna elements may be fit into an antenna enclosure with a size comparable to 3G and 4G base station antennas of today. To achieve a reasonable link budget, a typical example of a total antenna array size is comparable to an A4 sheet of paper.
The beams are typically highly directive and give beamforming gains of 20 decibels (dB) or more since so many antenna elements participate in forming a beam. This means that each beam is relatively narrow in horizontal and/or azimuth angle, a Half Power Beam Width (HPBW) of 5 degrees is not uncommon. Hence, a sector of a cell may need to be covered with a large number of potential beams. Beamforming can be seen as when a signal is transmitted in such a narrow HPBW that it is intended for a single wireless device or a group of wireless devices in a similar geographical position. This may be seen in contrast to other beam shaping techniques, such as cell shaping, where the coverage of a cell is dynamically adjusted to follow the geographical positions of a group of users in the cell. Although beamforming and cell shaping use similar techniques, i.e., transmitting a signal over multiple antenna elements and applying individual complex weights to these antenna elements, the notion of beamforming and beams in the embodiments described herein relates to the narrow HPBW basically intended for a single wireless device or terminal position.
In some embodiments herein, a system with multiple transmission nodes is considered, where each node has an array antenna capable of generating many beams with small HPBW. These nodes may then for instance use one or multiple LTE-Nx carriers, so that a total transmission bandwidth of multiples of hundreds of MHz can be achieved leading to downlink peak user throughputs reaching as much as 10 Gigabytes (Gbit/s) or more.
In LTE access procedures, a UE may first search for a cell using a cell search procedure, to detect an LTE cell and decode information required to register to the cell. There may also be a need to identify new cells, when a UE is already connected to a cell to find neighbouring cells. In this case, the UE may report the detected neighbouring cell identity and some measurements, to its serving cell, as to prepare for a handover. In order to support cell search, a unique Primary Synchronization Signal (PSS) and Secondary Synchronization Signal (SSS) may be transmitted from each eNB. The synchronizations signals are used for frequency synchronization and time synchronization. That is, to align a receiver of wireless device, e.g., the UE, to the signals transmitted by a network node, e.g., the eNB. The PSS comprises information that allows the wireless device in LTE to detect the 5 ms timing of the cell, and the cell identity within the cell-identity group. The SSS allows the wireless device in LTE to obtain frame timing and the cell-identity group. The PSS may be constructed from a Zadoff-Chu sequence of length 63, mapped to the center 64 subcarriers where the middle, so called DC subcarrier is unused. There may be three PSS in LTE, corresponding to three physical layer identities. The SSS may be constructed from two interleaved M-sequences of length 31 respectively, and by applying different cyclic shifts of each of the two M-sequences, different SSS may be obtained. In total, there may be 168 valid combinations of the two M-sequences, representing the cell identity groups. Combining the PSS and SSS, there may be thus in total 504 physical cell identities in LTE.
When a cell has been found, the UE may proceed with further steps to be associated with this cell, which may then be known as the serving cell for this UE. After the cell is found, the UE may read System Information (SI) in e.g., the Physical Broadcast CHannel (PBCH), known as the Master Information Block (MIB), which is found in a time frequency position relative to the PSS and SSS locations. The SI comprises all the information needed by a wireless device to access the network using a random access procedure. After the MIB is detected, the System Frame Number (SFN) and the system bandwidth are known. The UE may let the network know about its presence by transmitting a message in the Physical Random Access CHannel (PRACH).
When a cell has multiple antennas, each antenna may transmit an individual encoded message to the wireless device or UE, thereby multiplying the capacity by the number of layers transmitted. This is well known as MIMO transmission, and the number of layers transmitted is known as the rank of the transmission. Beamforming, traditionally, is equivalent to a rank 1 transmission, where only one encoded message is transmitted, but simultaneously from all antennas with individually set complex beamforming weights per antenna. Hence, in beamforming, only a single layer of Physical Downlink Shared CHannel (PDSCH) or Evolved Physical Downlink Control CHannel (EPDCCH) is transmitted in a single beam. This beamforming transmission is also possible in LTE, so after a UE has been associated with a cell, a set of N=1, 2, 4 or 8 Channel State Information Reference Signals (CSI-RS) may be configured for measurement reference at the UE, so that the UE may report a preferred rank 1 N×1 precoding vector containing the complex beamforming weights based on the CSI-RS measurement. The precoding vector may be selected from a codebook of rank 1 precoding vectors. In Rel-8, there are 16 rank 1 precoding vectors defined, and in Rel-12 a new codebook was designed with 256 rank 1 precoding vectors.
A “beam” may thus be the result of a certain precoding vector applied for one layer of transmitted signal across the antenna elements, where each antenna element may have an amplitude weight and a phase shift in the general case, or equivalently, the signal transmitted from the antenna element may be multiplied with a complex number, the weight. If the antenna elements are placed in two or three dimensions, and thus, not only on a straight line, then two dimensional beamforming is possible, where the beam pointing direction may be steered in both horizontal and azimuth angle. Sometimes, also three Dimensional (3D) beamforming is mentioned, where also a variable transmit power has been taken into account. In addition, the antenna elements in the antenna array may consist of different polarizations, and hence it is possible, by adjusting the antenna weights, to dynamically alter the polarization state of the transmitted electromagnetic wave. Hence, a two dimensional array with elements of different polarizations may give a large flexibility in beamforming, depending on the antenna weights. Sometimes, a certain set of precoding weights are denoted as a “beam state”, generating a certain beam in azimuth, elevation and polarization as well as power.
The most flexible implementation may be to use a fully digital beamformer, where each weight may be applied independent of each other. However, to reduce hardware cost, size and power consumption, some of the weighting functionality may be placed in hardware, e.g., using a Butler matrix, whereas other parts may be controlled in software. For instance, the elevation angle may be controlled by a Butler matrix implementation, while the azimuth angle may be controlled in software. A problem with the hardware beamforming may be that it involves switches and phase shifters, which may have some switching latency, making instant switching of beam unrealizable.
The PBCH is transmitted using the Common Reference Signals (CRS) as a demodulation reference. Since the PSS, SSS and the PBCH channel are intended for any UE that wishes to attach to the cell, they are typically transmitted in a cell broad coverage, typically using e.g., 120 degree sectors. Hence, such signals are not beamformed in LTE, as it is a risk that, e.g., the PSS and SSS will be in the side lobe or even in a null direction of the beamforming radiation pattern. This would lead to failure in synchronizing to the cell, or failure in detecting MIB.
Existing methods for transmission of synchronization signals from a network node to a wireless device are designed for wide area coverage at lower carrier frequencies of transmission than those expected to be used in future systems. These current methods may lead to numerous synchronization failures when used in communication systems using high frequency carriers, such as those projected to be used in the future 5G system.